Bite Size Med: Proximal Convoluted Tubule

Overview of Nephron Structure

• The kidney contains millions of nephrons.
• Each nephron consists of a glomerulus and a renal tubule.
• Parts of the renal tubule:
  • Proximal Convoluted Tubule (PCT)
  • Loop of Henle
  • Distal Convoluted Tubule
  • Collecting Duct

Function of the Proximal Convoluted Tubule

• Main Role: Maximum reabsorption of solutes and water.
• Reabsorption Stats:
  • 65% of sodium and water.
  • Other solutes include glucose, phosphate, amino acids, and bicarbonate.
• Structural Characteristics:
  • Presence of brush border to increase surface area.
  • High mitochondria count for energy in solute transport.
  • Looser tight junctions compared to distal nephron parts.

Early vs Late Proximal Convoluted Tubule

• Early PCT:

  • Reabsorbs sodium with glucose, amino acids, phosphate (co-transport with sodium).
  • Sodium-Glucose Co-Transporter (SGLT), predominantly type 2.

• Energy Source:

  • Sodium-Potassium ATPase pump at the basolateral membrane.
  • Creates concentration gradient for secondary active transport.

• Late PCT:

  • Sodium reabsorbed with chloride due to decreased glucose.

Glomerulotubular Balance

• Balance maintains 65% sodium reabsorption regardless of filtered load.
• Equal passive reabsorption of water maintains isosmotic conditions (300 mOsm/L).

Glucose Transport

• Transport Maximum:
  • Limit on carrier-mediated reabsorption.
  • Threshold for glucose appearance in urine.
• Uncontrolled Diabetes Mellitus:
  • High plasma glucose can exceed transport maximum, leading to glucosuria.

Bicarbonate Transport

• Important for urine acidification.
• Mechanism:
  • Carbon dioxide + water with carbonic anhydrase → carbonic acid → hydrogen ions + bicarbonate.
  • Bicarbonate reabsorbed with sodium (co-transport).
  • Hydrogen ions exchanged for sodium via sodium-hydrogen exchanger.
• Reabsorption Rate: 80% reabsorbed.

Secretion in PCT

• Although primarily reabsorptive, PCT also secretes:
  • Organic acids and bases (e.g., bile salts, urates).
  • Para-aminohippuric acid (PAH), used to measure renal plasma flow.

• Conclusion: The proximal convoluted tubule plays a critical role in reabsorbing vital solutes and managing acid-base balance.
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